The present invention relates to an apparatus and method for desolvating and selectively transmitting ions, based on the ion focussing principles of high field asymmetric waveform ion mobility spectrometry, for introduction into a mass spectrometer.
High sensitivity and amenability to miniaturization for field-portable applications have helped to make ion mobility spectrometry an important technique for the detection of many compounds, including narcotics, explosives, and chemical warfare agents (see, for example, G. Eiceman and Z. Karpas, Ion Mobility Spectrometry (CRC. Boca Raton, Fla. 1994); and Plasma Chromatography, edited by T. W. Carr (Plenum, N.Y., 1984)). In ion mobility spectrometry, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are gated into the drift tube and are subsequently separated based upon differences in their drift velocity. The ion drift velocity is proportional to the electric field strength at low electric fields (e.g., 200 V/cm) and the mobility, K, which is determined from experimentation, is independent of the applied field. At high electric fields (e.g. 5000 or 10000 V/cm), the ion drift velocity may no longer be directly proportional to the applied field, and K becomes dependent upon the applied electric field (see G. Eiceman and Z. Karpas, Ion Mobility Spectrometry (CRC. Boca Raton, Fla. 1994); and E. A. Mason and E. W. McDaniel, Transport Properties of Ions in Gases (Wiley, N.Y., 1988)). At high electric fields, K is better represented by Kh, a non-constant high field mobility term. The dependence of Kh on the applied electric field has been the basis for the development of high field asymmetric waveform ion mobility spectrometry (FAIMS), a term used by the inventors throughout this disclosure, and also referred to as transverse field compensation ion mobility spectrometry, or field ion spectrometry (see I. Buryakov, E. Krylov, E. Nazarov, and U. Rasulev. int. J. Mass Spectrom. Ion Proc. 128. 143 (1993); D. Riegner, C. Harden, B. Carnahan, and S. Day, Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, Calif., 1-5 Jun. 1997, p. 473; B. Carnahan, S. Day, V. Kouznetsov, M. Matyjaszczyk, and A. Tarassov, Proceedings of the 41st ISA Analysis Division Symposium, Framingham, Mass., 21-24 Apr. 1996, p. 85; and B. Carnahan and A. Tarassov, U.S. Pat. No. 5,420,424). Ions are separated in FAIMS on the basis of the difference in the mobility of an ion at high field Kh relative to its mobility at low field K. That is, the ions are separated because of the compound dependent behaviour of Kh as a function of the electric field. This offers a new tool for atmospheric pressure gas-phase ion studies since it is the change in ion mobility and not the absolute ion mobility that is being monitored.
An instrument based on the FAIMS concept has been designed and built by Mine Safety Appliances Company of Pittsburgh, Pa. (xe2x80x9cMSAxe2x80x9d) for use in trace gas analysis. The MSA instrument is described in U.S. Pat. No. 5,420,424 and is available under the trade mark FIS (for Field Ion Spectrometer). While the use of the MSA instrument (and similar instruments based on the FAIMS concept) for trace gas analysis is known, the inventors believe that they have identified certain heretofore unrealized properties of these instruments which make them more versatile.
The realization of these properties has resulted in the development of an invention which is designed to extend the functionality of the MSA instrument (and similar instruments based on the FAIMS concept). A summary and detailed description of the present invention is provided below.
In a first aspect, the present invention provides an apparatus for selectively transmitting and determining the mass to charge ratio of ions, comprising:
a) at least one ionization source for producing ions;
b) a high field asymmetric waveform ion mobility spectrometer, comprising:
i) an analyzer region defined by a space between first and second spaced apart electrodes, said analyzer region having a gas inlet at a first end and a gas outlet at a second end for providing, in use, a flow of gas through said analyzer region, said analyzer region having an ion inlet and an ion outlet, said ion inlet introducing a flow of ions produced by said ionization source into said analyzer region and said ion outlet allowing extraction of ions from said analyzer region;
ii) an electrical controller connectable to said electrodes and capable of applying an asymmetric waveform voltage and a direct-current compensation voltage to selectively transmit a type of ion in said analyzer region between said electrodes at a given combination of asymmetric waveform voltage and compensation voltage; and
c) a mass spectrometer having a sampler orifice, said sampler orifice being positioned proximate to said ion outlet to receive said selectively transmitted ions for analysis within said mass spectrometer.
Preferably, the first and second electrodes comprise curved electrode bodies and provide a non-constant electric field therebetween, said ions being selectively focussed in a focussing region created between said curved electrode bodies in said analyzer region.
In another aspect, the present invention provides apparatus for desolvating and selectively transmitting ions, comprising:
a) at least one electrospray ionization source for producing ions from a sample in liquid phase;
b) a high field asymmetric waveform ion mobility spectrometer, comprising:
i) an analyzer region defined by a space between first and second spaced apart electrodes, said analyzer region being in communication with a gas inlet, a gas outlet, an ion inlet and an ion outlet, said ion inlet introducing a flow of said ions into said analyzer region, and said ion outlet allowing extraction of ions from said analyzer region;
ii) a source of gas for providing a gas flow into said gas inlet and within said analyzer region, and out of said gas outlet, at least some of the gas flow being counter-current to said flow of ions being introduced at said ion inlet; and
iii) an electrical controller connectable to said electrodes and capable of applying an asymmetric waveform voltage and a direct-current compensation voltage to selectively transmit a type of ion in an analyzer region between said electrodes at a given combination of asymmetric waveform voltage and compensation voltage.
In yet another aspect, the present invention provides an apparatus for selectively transmitting ions, comprising:
a) at least one electrospray ionization source for producing ions from a sample in liquid phase; and
b) a high field asymmetric waveform ion mobility spectrometer, comprising:
i) an analyzer region defined by a space between first and second spaced apart electrodes, said analyzer region being in communication with a gas inlet and a gas outlet for providing, in use, a gas flow through said analyzer region, an ion inlet and an ion outlet, said ion inlet introducing a flow of ions produced by said electrospray ionization source into said analyzer region, and said ion outlet allowing extraction of ions from said analyzer region; and
ii) an electrical controller connectable to said electrodes and capable of applying an asymmetric waveform voltage and a direct-current compensation voltage to selectively transmit a type of ion in an analyzer region between said electrodes at a given combination of asymmetric waveform voltage and compensation voltage;
wherein, said electrospray ionization source is positioned external to said inner electrode so as reduce the effect of said asymmetric waveform voltage on said electrospray ionization source.
In yet another aspect, the present invention provides a method for desolvating and selectively focussing ions produced by electrospray ionization for introduction into a mass spectrometer, comprising the steps of:
a) providing at least one electrospray ionization source for producing ions from a sample in liquid phase;
b) providing an analyzer region defined by a space between first and second spaced apart electrodes, said analyzer region being in communication with a gas inlet, a gas outlet, an ion inlet and an ion outlet;
c) providing a gas flow into said gas inlet, and within said analyzer region, and out of said carrier gas outlet, at least some of said gas flow being counter-current to ions being introduced at said ion inlet;
d) providing an electrical controller connectable to said electrodes and capable of applying an asymmetric waveform voltage and a direct-current compensation voltage, to at least one of said electrodes;
e) adjusting said asymmetric waveform voltage and said compensation voltage to selectively focus a type of ion; and
f) extracting said selectively transmitted ions from said analyzer region at said ion outlet for introduction into a sampler cone of a mass spectrometer.